1. Field of the Invention
This invention relates to a method of improving the wear quality of ultra-high molecular weight polyethylene.
2. Description of the Prior Art
In a paper published in PLASTICS AND RUBBER PROCESSING AND APPLICATIONS II (1982)(215-224), "The Production of Orientated Polymers by Hydrostatic Extrusion" by B. Parsons and I. M. Ward, there is a discussion on small and large scale hydro-static extrusion experiments on a range of polymers with particular reference to polyethylene and polyoxymetholene. Hydro-static extrusion is shown to be a viable forming process for polymeric material and considerable property enhancement is said to be achieved. In this paper it is stated to be known that appreciable improvements in the stiffness of polymers can be produced by a high degree of plastic deformation, for example by stretching. There is reference to synthetic fibers, such as Nylon (polyamide) or polyethylene terethphthlate (polyester) being treated with a draw ratio (the ratio of final stretch length of the filaments to the initial length) which is comparatively small (typically about 5).
The paper goes on to state that it was considered that the draw ratio set a limitation to the degree of molecular alignment and corresponded to stretching the network of polymer molecules to some geometric limit--the "natural draw ratios"--after which fracture of the chains occurred.
A process is described in which the polymer is extruded into a tube shaped product under hydro-static pressure, which involves passing material over a fixed mandrel and a semi-conical plug for the purpose of imposing some hoop expansion to part of the billet material. There is reference to the extruded materials showing a marked improvement in thermal properties and other advantages. Some of the advantages are said to be increased stiffness, reduction in creep. Improved chemical resistance, low permeability to gases, high melting temperature, reduced shrinkage, very low axial expansion coefficient and high axial thermal conductivity are found, but it goes on to state that there may be possible defects, one of which is said to be deterioration in dry wear properties.
GB 2 060 469 relates to the deformation of thermoplastic polymers, to the solid phase deformation and concomitant orientations of orientatable thermoplastic polymers. There is reference to drawing an article through a die so as to induce a substantial degree of molecular orientation throughout the drawn article. There is also reference to a deformation ratio which is said to be the ratio of the cross-sectional area of the polymer billet to that of the extruder die orifice and in a discussion of U.K. Patent 1 480 479 there is reference to deformation ratios greater than about 8:1. GB 2 060 469 itself is related to a process for drawing the workpiece through the die so that its plastics strain is progressively increased during start up of the process. It is stated that the process produces a product with increased Young's modulus; resistance to creep; resistance to gas transport; enhanced deadfold; of axial thermal conductivity and it further states that the polymer should have a weight average molecular weight less than 1,000,000. The specification goes on to state that the die drawing process can be used as a forming process for polymers with higher molecular weight, but that this will not produce the enhanced properties listed.
In this specification, deformation ratio is used to define the ratio of the initial cross-sectional area of the workpiece to the final cross-sectional area of the product.
In the description of FIG. 1 it states that the polymer coating is drawn through a converging die and deformation of the polymer continues beyond the drawing block for some distance before it becomes frozen out. Thus, is this arrangement the molecular orientation is probably lengthwise in the produce or parallel to the draw direction. The advantages of the process are defined in all the examples by referring to the Young's modulus of the material achieved.
GB 2 156 733 relates to a process for solid phase deformation of tubular materials of an orientatable, thermoplastic polymer and to the orientated tubular material so produced.
This invention is intended to provide improved mechanical properties, especially in the directions other than the machine direction, that is the drawing direction, and there is reference to the drawing of a hollow workpiece through a die and simultaneously over an internally positioned former. Again, the specification states that when it is intended to produce a product with enhanced Young's modulus; resistance to creep; and resistance to gas transport; enhanced deadfold; or enhanced axial thermal conductivity, then the workpiece should desirably comprise a polymer having a weight average molecular weight of less than 1,000,000. As in the previously referred to G 2 060 469, the specification states that the process may be used as a forming process but not providing the enhanced properties listed above and it could, for example, be used with an ultra-high molecular weight polymer having a molecular weight of about 3,000,000.
Very clear orientated polyester material is referred to using a nominal deformation ratio of at least 2:1 and preferably at least 3:1 and there is reference to the clarity of the workpiece.
GB 2 225 551 refers to a similar process for producing a biaxially oriented tubular material in which the deformation is carried out in the absence of any external force acting in a direction which is perpendicular to the axis of the workpiece. In this specification there is reference to hoop draw ratio which is defined as a ratio of the final hoop dimension to the initial hoop dimension, (i.e. measured circumferentially) and the axial draw ratio is the ratio of the initial bulk cross-sectional are of the hollow workpiece to the final bulk cross-sectional area of the product. The specification states that for polymeric polyethylene the inner hoop draw ratio is at least 1.2 and more preferably at least 1.5 and most preferably at least 2 and the preferred axial draw ratio is at least 2 and preferably greater than 3. It also states that the outer draw ratio may be less than 1 but is preferably at least 1 and more preferably at least 1.5 or 2 and the ratio of the axial draw ratio to the inner hoop draw ratio is preferably at least 1 and less than 4, most preferably less than 2. The specification also refers to the use of polyethylene polymers having a weight average molecular weight of from 50,000 to 150,000 and also to polymers above 300,000.
Ultra high molecular weight polyethylene is used as a bearing material for various applications, for example in the bearing surfaces of the joints in prostheses. Various orthopedic manufacturers have sought methods of improving the wear properties and reducing the amount of generated wear debris from the bearing surfaces of such joints. It has been the intention of these projects to explore "improved" polyethylenes especially those produced by post processing. A number of potential improvements have been seen in recent years on the market, some of which have already failed to fulfil clinical expectations, for example carbon reinforced polyethylene, and cross-linked polyethylene.